In radio frequency (RF) communication applications, a RF receiver or a receiver portion of an RF transceiver is an electronic circuit that receives an input from an antenna, and uses electronic filters to separate a radio signal of interest from all other radio signals received by the antenna. An amplifier amplifies the radio signal of interest to a level suitable level for processing. A local oscillator generates a local oscillator signal for a mixer to convert the amplified the radio signal of interest to a different frequency. The converted frequency signal is further filtered, amplified, demodulated, and decoded into a usable form, such as sound, pictures, digital data, measurement values, navigational positions, etc.
A RF receiver may be required to tolerate the presence of large interfering signals lying within a pass-band that corresponds to a communication channel of interest. These interfering signals may have originated from users in adjacent channels and/or from transmission sources which may be relatively far removed in frequency from the channel of interest but whose large transmission power may still cause significant interference problems. These interfering signals may be referred to as blockers and their relative frequency and/or detected power to that of the desired signal may vary based on transmission scheme and/or operational conditions. The effect of interfering signals in the channel of interest may result in, for example, bit error rate (BER) degradation in digital RF systems and audible and/or visible signal-to-noise ratio (SNR) degradation in analog RF systems.
The ability to provide an interference-tolerant RF receiver design may be difficult to accomplish as second-order and/or third-order distortion effects are increasingly becoming a limitation in circuitry utilized by the wireless receivers. For example, mixers and/or other circuitry which may be employed to down-convert a frequency signal of interest to a zero intermediate frequency (IF) signal or to a low IF signal may generate, as a result of second-order and/or third-order nonlinearities, spectral components from blocker signals, which may be at or near direct current (DC). An effect of these spectral components may be to introduce a DC offset to the desired signals at the zero IF which may result in signal saturation or, as mentioned above, a noticeable degradation to the system's noise performance, such as represented by noise figure.
The concepts of linearity and noise figure are well known to those skilled in the art of RF receiver design. The second-order and/or third-order nonlinearities are typically represented by second and third intercept points (IP2 and IP3). Noise figure (NF) is a measure of degradation of signal to noise ratio (SNR), caused by components in the RF receiver. The noise figure is a ratio of an output noise power of a RF receiver to a portion thereof attributable to thermal noise in the input termination at standard noise temperature T0 (usually 290 degrees Kelvin). Thus, the noise figure is the ratio of actual output noise to that which would remain if the RF receiver itself did not introduce noise.
A duty cycle of the local oscillator signal affects the noise figure and the linearity (e.g., IP2) of the RF receiver. Traditionally, IP2 calibration of a RF receiver is performed by adjusting a mixer bias voltage in a manufacturing environment using test equipment to improve the linearity of the RF receiver.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art of RF receiver designs, through comparison of such systems with some aspects of the present invention, as set forth in the remainder of the present application with reference to the drawings.